Patch exprtk on Clang

[tresf]

Attempts to fix the Travis-CI environment by patching exprtk to workaround a bug with Clang.

More information about the bug is here: #3965 (comment)

[lukas-w]

@tresf I get this error on the fix/msvc branch (#4000), even though it includes your patch: https://travis-ci.org/LMMS/lmms/jobs/305976591#L7078

/Users/travis/build/LMMS/lmms/plugins/Xpressive/exprtk/exprtk.hpp:1965:64: error: default initialization of an object of const type 'const typename numeric::details::number_type<double>::type' (aka 'const exprtk::details::numeric::details::real_type_tag') without a user-provided default constructor

         const typename numeric::details::number_type<T>::type num_type;

                                                               ^`

Any idea what's going wrong here?

[tresf]

Just a formal follow-up...

As discussed on Discord, it appears Clang is now AppleClang and the fix is to use MATCHES instead of the current STREQUALS in plugins/Xpressive/CMakeLists.txt. This was likely caused by an XCode upgrade on the Travis-CI side.

https://github.com/tresf/lmms/blob/ffaba04be1a54308d7c7adccaf2397b1a92a3c78/plugins/Xpressive/CMakeLists.txt#L13-L14

Also note, upstream has plans to permanently fix this, so the workaround should be temporary.